The present invention pertains to the optimization of switched reluctance (SR) machines by determining various performance parameters and analyzing those parameters to optimize the operation of the machine.
The present invention relates to generators as well as to motors. To simplify the description, what follows pertains to a motor with the understanding invention also relates to generators. A switched reluctance (SR) motor comprises a ring shaped stator that has a plurality of pole positions defined by wire coils and disposed in a circular arrangement, a rotor disposed rotatably in an inner space defined by the stator having pole protrusions that face the pole portions of the stator. Normally, the rotor is an iron core formed by stacking iron plates. When supplied with electricity, the wire coils become magnetized together with the pole portions to attract the pole protrusions of the rotor. The rotor is continuously rotated by selectively distributing electric current to the coils.
Switched reluctance motors generate torque by utilizing a magnetic attractive force that acts between the rotor and the stator when the coils are magnetized. Conventional SR motors normally suffer vibration problems. The magnetic attractive force acting between the stator and the rotor is generally circumferential in direction and quickly increases as the rotor rotates during a cycle of the electric current distribution (to selected coils) and then abruptly terminates at the time of switching the current distribution (to other coils). This on-off cycle of magnetic attractive force causes the rotor and the stator to vibrate in generally circumferential directions. See U.S. Pat. No. 5,747,912.
A conventional switched reluctance motor is shown in FIG. 32 of U.S. Pat. No. 5,880,549 which comprises a four pole rotor where in each of the poles protrudes cross-wisely around the rotating shaft and a six-pole stator arranged around the rotor, each of the protruding poles of the stator having a concentrating winding. The current passes through the winding is unidirectional and includes a distorted wave current and direct current component.
Other switched reluctance motors are described in U.S. Pat. Nos. 6,028,385, 6,014,012, 6,002,233, 5,945,761 and 5,900,712; all of which are here by incorporated by reference.
In the field of electric machines of the Switched Reluctance type, it is well known that the machine performance (efficiency, output torque, torque ripple, etc.) is a function of when the current is turned-on and turned-off. (So-called turn-on and turn-off angles). It is also well known that the best angles will be different depending on speed, voltage, etc. Therefore others have looked for ways to take advantage of this to optimally design Switched Reluctance Motors (SRM). So far, however attempts have been confined to the following:
1. Approach based on optimization of one function at a time. [1-2]; This consists of an analytical or numerical optimization of one performance quality (efficiency, output torque, torque ripple, torque per ampere, energy, etc.) at a time. Because of the complexity of the physics and mathematics involved, however, people have only optimized with one parameter (say, efficiency) with at the most minor fine-tuning around the chosen point to improve together parameters (reduce torque ripple for instance). For example, reference (1) maximizes the Torque per Ampere only while reference (2) minimizes the energy consumption, by means of the root-mean-square (rms) phase current.
References:
[1] B. Fahimi et al. xe2x80x9cSelf-Turning of Switched Reluctance Motors for Optimized Torque per Ampere at all Operating Pointsxe2x80x9d. Proceedings of IEEE Applied Power Electronics Conference, APEC ""98, pp. 778-783, Feb. 1998.
[2] Ph. C. Kjaer et al. xe2x80x9cA New Energy control Strategy for Switched Reluctance Motorsxe2x80x9d. IEEE Transactions on Industry Applications, Vol. 31, No. 5, pp. 1088-1095, Sep./Oct. 1995.
2. Empirical approach (Testing of an existing prototype); Because of measurement limitations, one has essentially optimized for single performance quantity. This method requires that a prototype has already been built. In published literature, people usually do not specify explicitly that an empirical approach was used. Specific statements giving numbers without theoretical justification often mean that this kind of approach was used [3].
Reference:
[3] K. M. McLaughlin, M. W. Gluch. xe2x80x9cMethod and Apparatus for Controlling an Electric Assist Steering System using Two-dimensional Interpolation for Current Commandsxe2x80x9d. U.S. Pat. No. 5,475,289. Dec. 12, 1995.
It is an object of the present invention to provide a method of optimizing the performance parameters for a switched reluctance motor by obtaining the parameters of at least three objectives of the motor, plotting the parameters on a two or three dimensional chart and then mapping all of the parameters onto one chart.
Described is a method of optimizing performance parameters of a switched reluctance motor comprising ascertaining the parameters of at least three objectives of the motor, each as a function of at least one parameter.
Described is a method of optimizing performance parameters of a switched reluctance motor comprising ascertaining the parameters of at least three objectives of the motor selected from the group consisting of firing angles, power on the shaft of the motor, drive efficiency, torque ripple coefficient, output torque, torque per rms (root-mean-square) current per cycle, torque per mean ampere, energy consumption, phase target current level, hysteresis band size, duty cycle, DC voltage and zero-volt control loop, and the like;
plotting the parameters on an x-y (2 Dimension) or x-y-z chart (3 Dimension); and
mapping all of the desired parameters on to one chart and thereby ascertaining the optimum performance of the at least three parameters.
Also described is a method of operating a switched reluctance motor by ascertaining the optimum performance parameters as described above and operating the switched reluctance motor utilizing those parameters.